Killer whales, which are normally associated with the colder seas around the poles, are being found increasingly in UK waters. Scientists believe the creatures are being attracted by Britain's recovering fish stocks. Groups of up to 100 have been recorded off the coast by researchers.

Already this year, the creatures, also known as orcas, have been recorded as far south as the Isles of Scilly, and in the English Channel, off Folkestone. Other sightings this year have been in the North Sea, off Hartlepool, and in the Irish Sea, off the Welsh coast.

As well as eating fish, killer whales regularly hunt seals. Although attacks on humans are rare, experts warn against getting too close. Researchers from the University of Aberdeen and the Sea Mammals Research Unit, at St Andrews, are currently monitoring the population off Scotland, to test theories that the increase in sightings is down to a growth in numbers, and not simply better recording.

Andy Foote, from the University of Aberdeen, has, this summer, been studying numbers found off the Shetland Islands, where the pods of up to 100 have been seen.

"That sort of sighting does seem to be on the increase," he said. "The killer whales shift their migration and distribution quite drastically. Fish like herring and mackerel seem to be doing pretty well at the moment, and it makes sense for the killer whales to follow them.

"So in areas where you haven't seen killer whales before, all of a sudden, you are starting to see them. You see more up north, but you do get them turning up further south."

Since the 1950s and 1960s, when Britain's fish stocks began to collapse, few killer whales have been seen in UK waters. No records exist for earlier years, but scientists say there is a possibility the creatures were once more abundant. By comparing sightings from this year with photographs taken of killer whales over the past decade, the researchers have established, by using identifying marks on their fins, that the same ones are being seen repeatedly in UK waters.

Mr Foote added: "Until now, very little has been known about them in British waters. They have been considered as being transient and occasional animals that just move through the area. People thought they were very infrequent visitors. The fact that we are seeing the same ones year after year after year shows that that is wrong.

"Already we have highlighted that we have populations which are resident here for long periods of time, coming back to the same place, year after year after year, while some seem to remain all year around. "Having seen them going after seals here, I certainly wouldn't recommend going too close."

Paul Harvey, from the Shetland Biological Records Centre, which is home to Britain's biggest population, said: "We are definitely seeing more. We know we've got the same animals returning and we have some occurring here throughout the winter. It is a relatively recent phenomenon. If you talk to fishermen, they just didn't used to see them. Now, they see them every time they haul their nets.

"Something has gone on, since about the 1990s, when we first started to see more. We don't know how many pods we are dealing with. That is the value of the new research."

Killer whales – actually the largest species of dolphin – are known to occur in all the world's oceans. Those near the Shetland come close to shore to feast on seals.

"They are more exciting to see than other species," Mr Harvey added. "You see more of them out of the water and they are often doing something, like hunting seals. They are really spectacular."

Rob Lott, from the Whale and Dolphin Conservation Society, said: "The north of the country has become a stronghold but they do turn up all around the UK. They are opportunistic and their distribution is driven by the prey available to them."

As well as fish and seals, killer whales will also feed on sharks, including great whites, and even other species of whale, including the blue whale – the world's largest animal. They can swim at speeds of up to 30mph and hunt in packs. They grow up to 30ft long and live up to 35 years. Adults eat around four per cent of their body weight each day, while young whales eat up to ten per cent.